Revision notes for AQA GCSE Chemistry Structure and formulae of alkenes. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Structure and formulae of alkenes
What you'll learn
- What makes a molecule an alkene.
- How to use the alkene general formula CnH2n.
- Why alkenes are described as unsaturated.
- How to recognise alkenes from names, molecular formulae and displayed formulae.
The bonding ideas you need first
In this topic, you are dealing with small molecules: groups of atoms joined together by covalent bonds. A covalent bond is a shared pair of electrons and is drawn as a line in a displayed formula.
For the alkenes you meet at GCSE:
- each carbon atom makes four bonds in total
- each hydrogen atom makes one bond
- a double bond, written C=C, counts as two bonds for each carbon atom involved
Key Idea
Bond-counting rule
In alkene displayed formulae, carbon must have four bonds in total and hydrogen must have one. The two lines in C=C count as two bonds, not one.
What is an alkene?
A hydrocarbon is a compound containing carbon and hydrogen atoms only.
Definition
Alkene
An alkene is a hydrocarbon containing a carbon-carbon double bond, written C=C. The simple alkene homologous series in this GCSE topic has one C=C double bond in each molecule.
The C=C bond is the key feature. If a molecule contains oxygen, chlorine or any element other than carbon and hydrogen, it is not a hydrocarbon, so it is not an alkene in this GCSE sense.
Example
Recognising an alkene from a formula drawing
Suppose you are given CH3–CH=CH2.
- Check the elements present: it contains only carbon and hydrogen, so it is a hydrocarbon.
- Check the bonding: the equals sign between two carbon atoms shows a C=C double bond.
- Combine the two conditions: a hydrocarbon with a C=C double bond is an alkene, so CH3–CH=CH2 is an alkene.
Homologous series and the general formula
A homologous series is a family of compounds with the same functional group and a regular pattern in their formulae. A functional group is the atom, group of atoms or bond that gives a family of organic compounds its typical reactions.
For alkenes, the functional group is the C=C double bond.
The general formula for the alkene homologous series is:
CnH2n
Here, n means the number of carbon atoms in the molecule. The number of hydrogen atoms is double the number of carbon atoms.
Example
Finding the formula of pentene
- The ending -ene tells you pentene is an alkene, so use the general formula CnH2n.
- The prefix pent- means 5 carbon atoms, so n=5n = 5n=5.
- Calculate the number of hydrogen atoms: H=2n=2×5=10H = 2n = 2 \times 5 = 10H=2n=2×5=10, so pentene has the molecular formula C5H10.
Why alkenes are unsaturated
An alkane is a saturated hydrocarbon with only single carbon-carbon bonds. Alkanes have the general formula CnH2n+2.
Definition
Unsaturated
An unsaturated molecule contains at least one carbon-carbon double bond. Alkenes are unsaturated because they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
This happens because the C=C double bond uses up bonding positions that would otherwise be used to bond to hydrogen atoms.
Example
Comparing propene with propane
- Propane is an alkane with 3 carbon atoms. For an alkane, the number of hydrogen atoms is H=2n+2=2×3+2=8H = 2n + 2 = 2 \times 3 + 2 = 8H=2n+2=2×3+2=8, so propane is C3H8.
- Propene is an alkene with 3 carbon atoms. For an alkene, the number of hydrogen atoms is H=2n=2×3=6H = 2n = 2 \times 3 = 6H=2n=2×3=6, so propene is C3H6.
- Propene has 2 fewer hydrogen atoms than propane. That difference is linked to the C=C double bond, so propene is unsaturated.
Common Mistake
Mixing up alkane and alkene formulae
Alkanes use CnH2n+2; alkenes use CnH2n. The ending -ene should make you think of the C=C double bond.
The first four alkenes
You only need to know the names of the first four members of the alkene homologous series:
| Alkene | Number of carbon atoms | Molecular formula |
|---|---|---|
| Ethene | 2 | C2H4 |
| Propene | 3 | C3H6 |
| Butene | 4 | C4H8 |
| Pentene | 5 | C5H10 |
There is no “methene” in this series because a carbon-carbon double bond needs two carbon atoms.
Tip
Naming shortcut
The ending -ene tells you the compound is an alkene. The prefixes you need here are eth- for 2 carbons, prop- for 3, but- for 4 and pent- for 5.
Ways to represent alkene molecules
A molecular formula tells you the actual number of each type of atom in one molecule. For example, C3H6 tells you there are 3 carbon atoms and 6 hydrogen atoms in one molecule of propene.
A displayed formula shows every atom and every bond. This is useful because you can see the C=C double bond directly.
You may also see a shortened structural formula, such as CH2=CH–CH3. This groups hydrogen atoms with the carbon they are attached to, but still shows the important C=C double bond.
Example
Drawing the displayed formula of propene
- Use the name: prop- means 3 carbon atoms, and -ene means there must be a C=C double bond.
- Draw a chain of 3 carbon atoms with a double bond between two of them: C=C–C.
- Add hydrogens until each carbon has four bonds: the first carbon gets 2 hydrogens, the middle carbon gets 1 hydrogen, and the final carbon gets 3 hydrogens.
- Count the atoms to check: there are 3 carbon atoms and 6 hydrogen atoms, matching C3H6.
For butene and pentene, you do not need detailed names showing exactly where the double bond is. At this stage, focus on recognising that the molecule is a hydrocarbon and contains a C=C double bond.
Recognising alkenes quickly
In questions, you might be asked to recognise an alkene from a name, a molecular formula or a displayed formula.
Use these clues:
- Name ends in -ene → alkene.
- Molecular formula fits CnH2n → alkene formula pattern.
- Displayed formula shows C=C and only carbon/hydrogen atoms → alkene.
Example
Choosing which formula could be an alkene
Which of these could be an alkene: C4H8, C4H10, C3H6O?
- First check for hydrocarbons. C3H6O contains oxygen, so it is not a hydrocarbon and cannot be an alkene.
- For C4H8, there are 4 carbon atoms. An alkene should have H=2n=2×4=8H = 2n = 2 \times 4 = 8H=2n=2×4=8 hydrogen atoms, so C4H8 fits the alkene formula.
- For C4H10, there are 4 carbon atoms but 10 hydrogen atoms. That matches the alkane pattern CnH2n+2, not the alkene pattern, so C4H10 is not an alkene.
Exam technique
In the exam
- For a name, look at the ending first: -ene means alkene.
- For a molecular formula, check it is a hydrocarbon, then test whether the number of hydrogens fits H=2nH = 2nH=2n.
- For a displayed formula, spot the C=C double bond and check that the molecule contains only carbon and hydrogen.
- When drawing displayed formulae, count bonds carefully: carbon has four bonds and hydrogen has one.
Self review
Check yourself
- What feature must you see in a displayed formula for a molecule to be an alkene?
- What is the molecular formula of butene?
- Why does propene have two fewer hydrogen atoms than propane?
Reactions of alkenes and alcohols (chemistry only)
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